1. Field of the Invention
The present invention relates generally to beamsteered phased array antennas. More particularly, the present invention relates to an optoelectronic wide bandwidth photonically beamsteered phased array which utilizes a digital subarray interface in both the transmit and receive signal paths.
2. Description of Related Art
A beamsteered phased array is an antenna system comprising individual antenna elements in which the relative phases of the signals respectively radiated or received by the antenna elements control the effective beam pointing direction. The relative phases of the signals radiated by the individual antenna elements define the transmit beam, while the relative phases of the signals received by the individual elements define the receive beam. The beamsteering effect is substantially the same as that produced in a mechanical scan antenna by physically repositioning the antenna elements.
In a true time delay beamsteered phased array, known time delays introduced into the transmitted or received signals control the relative phase between individual elements and thereby the antenna beam pointing direction. U.S. Pat. No. 5,051,754 discloses a true time delay phased array utilizing a photonic true time delay circuit for both the transmit and receive signal paths. A plurality of fiber optic true time delay circuits receive an RF signal input from an RF exciter and output an optical signal having a controlled time delay. The optical signal is detected and amplified to provide an RF signal which serves as the transmit carrier signal and the local oscillator for the receive circuit mixers, and thus the appropriate delay is introduced on both transmit and receive.
The transmit signal path in the true time delay phased array described above includes an RF analog signal interface between the true time delay circuits and the transmit/receive modules. In the receive signal path, however, a multiplexed digital signal resulting from the analog to digital conversion and time delay of the signals received by the in-phase and quadrature mixers is transmitted across a fiber optic cable from the transmit/receive module to the radar processor. The control signal interface between the transmit/receive module and the radar processor is also digital.
A digital interface would also be desirable in the transmit signal path. All RF signals could then be confined to a limited area near the radiating elements, while all signal reference, processing and control circuitry could be remotely located. An entirely digital interface would provide numerous advantages including the ability to support wider instantaneous bandwidth signals, increased tolerance to alignment errors, reduced reliance on calibration, reduced RF interface signal loss and mismatch problems, simplified manufacturing and integration, increased design flexibility, low cross-talk and resistance to electromagnetic interference. In addition an all digital transmit and receive interface would permit the use of self-contained subarrays of transmit/receive modules, resulting in significant structural improvements in the RF hardware, such as size and weight reductions. Attaining the above advantages will permit more widespread use of phased arrays in size and weight sensitive applications such as aircraft skins, missiles, spacecraft and automobiles. Significant cost savings would also be possible in other traditional phased array applications, such as ship or ground-based radars.
However, there is no true time delay phased array presently available which incorporates a digital interface in both the transmit and receive signal paths. It is therefore not possible to obtain the above advantages using current technology. Although the phased array described above utilizes fiber optic cables to interface the true time delay circuits to the array modules in the transmit path, these cables carry analog signals and therefore many of the problems associated with other types of analog interfaces remain.
It is therefore an object of the present invention to provide a true time delay photonically beamsteered phased array which incorporates a digital interface in the transmit signal path as well as the receive and control signal paths, and which provides the advantages discussed above.